This circuit in this project takes input from a piezo diaphragm which is provided to non-inverting pin of op-amp in LM324 (pin3). The inverting input of the op-amp is grounded through R1 (1k). The op-amp is thus configured to be used in non-inverting mode. This means that the output is in phase with the input of op-amp. If someone tries to walk over a restricted area, the pressure variations of the feet are sensed by the piezo sensor which gives the analog output. This output is amplified by the op-amp and its gain depends on the input resistors R1 (1k) and R2 (3.3k) in accordance with the relation:G = (1 + R2/R1).
The output of op-amp is sufficient enough to drive a speaker, connected to its output, after getting filtered by a capacitor C1 (10uF).